Bong sau is a common problem for beginners....although it is not just beginners who have bong problems....even after 8 years of training my teacher still corrects my bong sau
A common tendency especially at the start is to lift the bong like a karate rising block..this is incorrect..it should not rise but spiral forward instead towards the enemys centre...this forward spiralling action will naturaly divert the enemys punch/bridge and when combined with a slight turn of the horse/torso will give you a clear line of attack with your rear hand (wu sau)
WSL(my teachers teacher or sigung in cantonese) said the action of bong can be compared to pouring tea into a tea cup...if bong is the tea pot...fook sau is the tea cup...that analogy may help you
Bare in mind also that wing chun is not so much about finished static positions as it is about the route we take to get there....therefore make sure your emphasis is on function rather than form i.e. that your first priority is to get the job done not to look good...that said a useful guide when learning how to correctly execute the action of bong sau is for the upper arm (humerus) to end up 90 deg to the body/torso, for the elbow to be about the same height as the shoudler, for the forearm to be at a 45 deg angle relative to the body and for the wrist to not be on the centre but a bit further over (not too much though).
However remember these are just general guidelines...you may have to make adjustments relative to the height of your opponent...the situation you find yourself...the kind of pressure your opponent gives you etc.
'In the woods there is always a sound...In the city aways a reflection.'
'What about the desert?'
'You dont want to go into the desert'
- Spartan